1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to a cathode material for a lithium electrochemical cell and, in particular, to a silver vanadium oxide cathode used in a lithium electrochemical cell designed for high current pulse discharge applications.
2. Prior Art
Silver vanadium oxide (SVO) cathode active materials can be synthesized from silver-containing compounds such as Ag, AgI, AgO, Ag2O, AgNO3, AgNO2, AgCO3, AgVO3, Ag(CH3CO2), and mixtures thereof, and from vanadium-containing compounds such as AgVO3, V2O5, V2O4, V2O31 V3O7, V4O9, V6O13, NH4VO3, and mixtures thereof under thermal treatment. Historically, two types of reactions are described. The first type is best described in U.S. Pat. Nos. 4,310,609 to Liang et al. and 5,545,497 to Takeuchi et al., which are assigned to the assignee of the present invention and incorporated herein by reference, as a decomposition reaction in which one or both of the starting materials is decomposed before formation of SVO. The second type is described in U.S. Pat. Nos. 5,221,453 to Crespi and 5,895,733 to Crespi et al. as a combination reaction in which the only elements involved in the reaction are silver, vanadium and oxygen. In all of the above patents, the starting materials are mixed in a predetermined stoichiometry before thermal treatment begins. The stoichiometric elemental ratio of silver to vanadium is calculated to be the ratio of silver to vanadium in the final SVO product. For example, if the final desired product is Ag2V4O11, the initial mixture of the silver-containing compound and the vanadium-containing compound is adjusted to a silver to vanadium molar ratio of 1 to 2. Therefore, the reactions described in all of the above patents are characterized as xe2x80x9cone step raw materials mixingxe2x80x9d reactions.
In both decomposition and combination reactions, the reactions are known to proceed in solid state or the reactions proceed in an inhomogeneous state to begin with. From a chemical point of view, the inhomogeneous reactions occur at the interface and they generally take longer than the homogeneous reactions to reach completion. In many cases, the end product is also inhomogeneous in terms of the chemical composition throughout the bulk of the material. Completeness of the reaction is dependent on the reaction temperature, reaction time, how well the reactive raw materials are mixed, the raw material particle size, and the like. Therefore, even though the correct stoichiometric ratio of silver to vanadium is used in the raw material mixture, the reaction product often ends up being a mixture of the desired product and several reaction intermediates.
For example, when SVO with a stoichiometry of Ag:V=2:4 is synthesized from AgNO3 and V2O5 at 375xc2x0 C., as described in the above-referenced patent to Liang et al., a product material with at least three components is obtained. The three components are distinguishable by differential thermal analysis (DTA) to be AgVO3 (silver rich component), Ag2V4O11 (xcex5-phase SVO, the desired product) and Ag1.6V4O10.8 (xcex3-phase SVO, silver deficient component).
Although this mixture is successfully used in the construction of Li/SVO cells for implantable defibrillator applications, it is difficult to control the quality of the synthesized material. The relative ratio of each component in the product active material changes based on reaction conditions such as starting material particle size, how well the materials are mixed, reaction batch size, reaction temperature, type of furnace used, reaction time, and the like. This difficulty is manifested in Li/SVO cell performance variations from one lot to the next.
One way to minimize component variations from lot to lot is, in theory, to ensure that the reaction proceeds to completion by either prolonging reaction time or increasing reaction temperature. Prolonging reaction time is undesirable due to its lack of efficiency as a production process. Increasing reaction temperature has been used as an alternative means of synthesizing improved SVO [R. A. Leising, E. S. Takeuchi, Chem. of Material, 5, 738-42, (1993); R. A. Leising and E. S. Takeuchi, Chem. of Material, 6, 489-95, (1994); and U.S. Pat. Nos. 5,895,733 and 5,955,218, both to Crespi at al.].
Nonetheless, achievement of a single component or pure phase SVO has not been demonstrated in any of the above-referenced prior art. In every case, a higher reaction temperature results in a more highly crystalline product material. In fact the X-ray powder diffraction patterns for synthesized SVO shown in FIGS. 2A, 2B and 2C of U.S. Pat. No. 5,895,733 to Crespi et al., in FIGS. 15A, 16A and 17 of U.S. Pat. No. 5,955,218 to Crespi et al. and by Leising et al. in FIGS. 3 and 4 in Chem. of Material, 6, 489-95, (1994) are essentially identical or very similar. However, as demonstrated by Leising et al., these materials contain multiple phases of components, such as Ag2V4O11 (xcex5-SVO) and Ag1.6V4O10.8 (xcex3-SVO) [FIG. 2 in Chem. of Material, 6, 489-95, (1994)].
Although several SVO phases have been discovered and extensively studied, it is believed that the prior art has not demonstrated or suggested the use a single phase SVO material in an electrochemical cell, especially a lithium cell, for implantable cardiac defibrillator applications. In other words, the stoichiometry of raw materials used in a particular synthesis cannot be used as an indicator that the product SVO material is in its pure phase. Therefore, there is still a need to fully understand the chemical pathways which produce SVO and to improve the chemical system to provide a desired single phase or nearly pure phase (enriched) active material.
According to the present invention, a pure phase SVO material, for example xcex3-SVO, is provided as well as an enriched xcex5-SVO material. These materials are in comparison to those of the prior art which include some of the starting materials in the product compound. The present invention is also directed to a study of the discharge characteristics of the enriched xcex5-SVO and the pure xcex3-SVO. Finally, a synthesis technique is disclosed for coating enriched xcex5-SVO with pure xcex3-SVO. In the present synthetic methodologies, the advantage of each constituent in the product SVO material is maintained while suppressing their disadvantages.
These and other aspects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.